1. Field of the Invention
The present invention relates to an ink jet print head that performs printing by ejecting ink from ejection ports formed in a substrate, and more specifically, to an ink jet print head with test terminals arranged on the substrate.
2. Description of the Related Art
Common ink jet print heads (hereinafter simply referred to as print heads) use electrothermal transducing elements or electromechanical transducing elements as elements generating energy required to eject ink. Such print heads apply pulsed electric energy to the transducing elements or instantaneously change potential to allow ink to be ejected at a driving frequency of several kHz to 100 kHz. In a print head using electrothermal transducing elements, a current of several to several hundred mA normally flows per element. Thus, switching is preferably performed using power transistors or the like. Elements for driving including transistors are formed on a silicon substrate during the same manufacturing process by which semiconductors are manufactured.
The operation of such an actual circuit is checked using a dummy circuit called a test element group (TEG) as in the case of semiconductors. However, the dummy circuit does not necessarily offer the same characteristics as those of the actual circuit to be measured, wires are preferably drawn out directly from the actual circuit and guided to test terminals for checks.
Such test terminals are not involved in actual driving of print head. Thus, as disclosed in Japanese Patent Laid-Open No. H07-323549 (1995), the test terminals are provided separately from terminals receiving printing signals, normally at positions where the test terminals avoid affecting the size of the substrate.
FIG. 8 is a diagram showing a print head 1001 which is an example of a conventional print head and which ejects pigment black ink. The print head 1001 includes a print element substrate 1008 consisting of a nozzle forming member 1033 and a substrate 1021 which formed a printing element that uses a heater as an energy generating element. The heater heats ink to allow ink droplets to be ejected under the action of film boiling. Furthermore, the print head 1001 includes an electric wiring substrate 1002 that transmits driving signals and the like from an ink jet printing apparatus, and an electric connection sealing portion 1007 that insulates and protects the electric connection between the print element substrate 1008 and the electric wiring substrate 1002.
FIG. 9 is a partly enlarged perspective view of a portion of the print head 1001 which ejects ink. For facilitation of description, the figure shows only a part of the electric connection sealing portion 1007 (a hatched portion in the figure corresponds to a cross section of the omitted portion). Heaters (not shown in the drawings), elements for driving (not shown in the drawings), test terminals 1025, and connection terminals 1022 are patterned on the substrate 1021; leads 1024 from the electric wiring substrate 1002 are joined to the connection terminals 1022. A nozzle forming member 1033 is provided on the substrate 1021 so as to form nozzles that communicate with ejection port 1028. An intermediate layer 1027 is provided between the nozzle forming member 1033 and the substrate 1021.
The test terminal is arranged at an end of the print element unnecessarily enlarging the size of the print element substrate 1008. The test terminals are also covered with the nozzle forming member or the intermediate layer 1027 and thus protected from ink. Moreover, as disclosed in Japanese Patent Laid-Open No. 2005-132102, a sealing compound 1030 is of a thermosetting type similarly to the electric connection sealing portion 1007. This prevents the ink from entering the test terminals.
FIG. 10 is a circuit diagram showing a part of a circuit on the substrate 1021 with the test elements. During driving, a current is passed through a terminal 1 (power source) and a terminal 2 (GND) to actuate the heater to cause bubbling and the subsequent ink ejection. On the other hand, one of the check items required to determine whether or not the substrate 1021 is acceptable is to determine whether or not an illustrated transistor portion exhibits a predetermined resistance value to provide proper driving. In this case, the resistance value is measured between test terminals A and B to directly measure the resistance value of the transistor portion to determine whether or not the substrate 1021 is acceptable.
However, the thermosetting epoxy resin making up the sealing compound 1030 and the electric connection sealing portion 1007 causes stress on the nozzle forming member under the action of heat during hardening. The stress may warp the nozzle forming member to peel off from the substrate 1021 an end of the nozzle forming member, resulting in a gap 1032. The test terminals 1025 are often arranged in the end of a substrate so as to prevent an increase in the size of the substrate 1021. Therefore the test terminals 1025 are often arranged in the vicinity of the position where the gap 1032 is created. Accordingly in some cases, the test terminals 1025 may be connected to the exterior via the very small gap and come into contact with ink or moisture. As shown in FIG. 10, the test terminal A has a high potential with respect to the ground (GND). There is a possibility that trouble occurs when ink or moisture touches the terminal A.
Furthermore, if the test terminals 1025 are covered with gold plating with a thickness of, for example, 5 μm and the intermediate layer 1027 is about 3 to 5 μm in thickness, then the intermediate layer on the gold-plated test terminals 1025 is only at most 2 μm in thickness. In this condition, when the nozzle forming portion is peeled off, partly because gold originally exhibits improper responsiveness, the intermediate layer 1027 on the test terminals 1025 may be peeled off together with the nozzle forming portion. As a result, the test terminals 1025 may be connected to the exterior via the very small gap and come into contact with ink or moisture. A trouble may be caused in the terminal A.
Moreover, it is assumed that the intermediate layer 1027 is adapted to provide the functions of an insulating layer. Then, if the intermediate layer 1027 on the gold-plated test terminals 1025 is broken, even when the broken part is filled with the sealing compound, ions may migrate through the nozzle forming member to affect the test terminals 1025. Such effect of the test terminals 1025 does not directly affect the ink jet print head. However, if any potential is applied to the print head as in the case of the test terminal A in FIG. 10, then the effect may infrequently propagate through the wiring in the substrate 1021 toward the part with the higher potential, thus damaging circuits or wires in the substrate 1021.